Purification and comparative characterization of the E. coli mec plus and RII-DNA-cytosine methylases will be continued. In vitro experiments will be carried out to demonstrate that the mec plus methylase can protect phage lambda DNA against degradation by the RII restriction endonuclease. The specificity of other enteric bacterial DNA cytosine methylases will also be screened. The host-specificity system in Salmonella typhimurium controlled by the SA-locus is under investigation. Experiments to determine the nature of the methylated base(s) involved in SA-modification are in progress. The question of DNA methylation in yeast will be studied with respect to the nature and levels of methylated bases in nuclear and mitochondrial DNA. These studies will be applied to both parental and radiation-sensitive mutant strains to determine whether specific methylation might protect radiation damaged DNA against repair nuclease(s). BIBLIOGRAPHIC REFERENCES: Hughes, S.G., and Hattman, S. (1975). The Sensitivity of bacteriophage lambda DNA to restriction by endonuclease RII. J. Mol. Biol. 98: 645.Schlagman, S., Hattman, S., May, M.S., and Berger, L. (1976). In vivo methylation by Escherichia coli mec plus DNA cytosine methylase protects against in vitro cleavage by the RII-restriction endonuclease (R-Eco RII). J. Bacteriol., in press.